ADSR [Attack/Decay/Sustain/Release]

When this envelope generator receives a gate input, it typically starts at 0 volts (which is the equivalent of silence when connected to a Voltage Controlled Amplifier, or the lowest frequency when connected to a voltage controlled filter or oscillator) and raises to the maximum voltage it can output (typically 5 to 10 volts depending on system; it can often be set with an output level control) over a time set by the Attack control. Once it reaches that level, the output voltage immediately starts dropping to speed set by the Decay control it until it reaches the voltage set by the Sustain control. If the input gate is still active, this level is maintained until the gate goes back to 0 volts (usually because you released the key on a controlling keyboard, etc.). At that time, the output voltage then starts dropping back to 0 volts at the rate set by the Release control.

Where ADSR designs differ is what happens when you don’t let them go through their entire cycle. For example, if you release a note before the Attack or Decay stage is finished, many envelopes jump straight to the Release stage, continuing on from whatever the current voltage level is; some envelopes go all the way through the time designated for Attack or Decay before moving onto Release. If you re-trigger the envelope by sending it a new gate = high signal, analog envelope generators usually start their Attack stage from the current voltage they were at; digital ones tend to suddenly jump to 0 volts and restart from there.